The field of the invention is magnetic resonance imaging (“MRI”) methods and systems and more particularly a method and apparatus for acquiring MRI data using a radial, or projection reconstruction, acquisition and correcting for in plane subject motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) the individual nuclei in the tissue attempt to align their magnetic moments with this polarizing field but as a result of nuclear spin, precess about it in random order at their characteristic Larmor frequency. The Larmor frequency is dependent on the strength of the magnetic field and on the properties of a particular nucleus as represented by a magnetogyric constant γ. Nuclei which exhibit this phenomenon are referred to as “spins”.
By convention, the polarizing field B0 is considered to lie along a z axis of a Cartesian coordinate system. The precession of the nuclei in the polarizing field B0 creates a net magnetic moment Mz in the direction of the polarizing field. Individual spins have magnetic moments that are perpendicular to the z axis in the transverse or x-y plane, however, the random orientation of the spins cancels any net transverse magnetic moment. In MRI imaging, a radio frequency signal is applied in the x-y plane near the Larmor frequency to tip the net magnetic moment into the x-y plane so that it rotates at the Larmor frequency. The practical value of this phenomenon resides in the signal which is then emitted by the excited spins termed the NMR signal (“nuclear magnetic resonance”).
An image of a patient may be obtained by evaluating the NMR signal contributed by different spins at different locations in the patient's tissue. A pulse sequence using gradient magnetic fields encodes location information on the spins in the form of the phase and frequency. The encoded spin signal may then be separated to produce an image. The pulse sequences most commonly found in clinical applications are so-called Cartesian, or Fourier, pulse sequences in that the phase encoding and frequency encoding gradient axes are orthogonal and each NMR signal sample is viewed as a sample from a row and column in Cartesian k-space.
A wide variety of such Cartesian pulse sequences is known. For example, the spin warp or spin echo technique is described in “Spin Warp NMR Imaging And Applications To Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, vol. 25, pp. 751-756 (1980); the steady state free precession (“SSFP”) technique including gradient refocused acquired steady state pulse sequences (“GRASS”) as described in U.S. Pat. No. 4,665,365 and contrast enhanced fast imaging (SSP-ECHO) described in “Rapid Fourier Imaging Using Steady State Free Precision”, R. C. Hawks and S. Patz, Magnetic Resonance in Medicine 4, pp. 9-23 (1987); and echo planer imaging (“EPI”) is described in an article by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). After the NMR signals are acquired, an image is reconstructed using a 2D or 3D Fourier transformation.
In more recent years an alternative method of data acquisition that had fallen out of favor is being used more widely in which the k-space data is filled not by rows and columns but by a series of radial projections about a point within k-space. This acquisition technique is analogous to the acquisition of data in an x-ray computed tomography (“CT”) machine and allows the data to be reconstructed into an image by CT-type algorithms including filtered back projection. This is known as a radial acquisition and is sometimes referred to as a projection reconstruction method. In fact, it was the method by which the first MR images were acquired and reconstructed by Paul C. Lauterbur in 1973. Such radial acquisition methods are disclosed, for example, in U.S. Pat. Nos. 6,630,828; and 6,188,922.
MRI acquisitions using a radial sampling trajectory and projection reconstruction (“PR”) are known to have intrinsic advantages over two-dimensional Fourier transform (2DFT) acquisitions when imaging a moving object. Motion artifacts in PR acquisitions manifest as radial streaks perpendicular to the direction of motion with diminished amplitude near the moving object. In 2DFT approaches, motion often creates ghosts in one direction with strongest intensity near the source of movement. Because of the improved motion characteristics of PR methods as well as other properties, interest in PR has increased recently for applications such as high-speed 3D imaging, MR angiography, dynamic imaging and fluoroscopy, catheter tracking and reduced field of view (FOV) imaging.
Many techniques have been proposed to further improve the robustness of PR against motion artifacts. Some techniques simply try to minimize the effect of motion with approaches such as respiratory ordered view angles (analogous to respiratory ordered phase encoding used in conjunction with 2DFT imaging) or with fast scanning. Other techniques impose consistency constraints on the PR data to reject or filter out inconsistent data (Glover G H, Noll D C, “Consistent Projection Reconstruction (CPR) Techniques For MRI,” Magn Reson Med 1993;29:345-351.) or to resort (Gai N, Axel L., “Correction Of Motion Artifacts In Linogram And Projection Reconstruction MRI Using Geometry And Consistency Constraints,” Med Phys 1996;23:251-262.). Others attempt to correct directly for the effect of translation by shifting projections by their centers of mass as described by Shankaranarayanan A, Wendt M, Lewin J S, Duerk J L, “Two-step Navigatorless Correction Algorithm For Radial k-Space MRI Acquisitions,” Magn. Reson. Med. 2001; 45(2):277-288, or correct translation and rotation by registering low resolution interleaved acquisitions as described by Schäffter T, Rasche V, Carlsen I C, “Motion Compensated Projection Reconstruction,” 1999;41(5):954-63.
While such prior “navigatorless” methods correct directly for in-plane translational motion of the subject during the scan, none directly detect subject rotation. Many subjects of clinical importance have complicated movements which include rotation, and correction for such movements is highly desirable. The approach by Gai and Axel attempts to resort projections to partially compensate for limited rotations occurring in the monotonic segments of the 2nd moment trajectory versus projection view angle. However, it has not previously been realized that the consistency properties of the 2nd moments of the spatial projections in radial MRI in conjunction with a specific projection angle acquisition time order can be used to directly detect unlimited in-plane rotation occurring at any point during the imaging acquisition.